系統日 - 年(One year minus on Date)

http://www.orafaq.com/forum/t/143512/2/

select add_months(sysdate,-12) from dual ; 

select sysdate - numtoyminterval(1, 'YEAR') from dual;

SELECT SYSDATE - interval '1' YEAR FROM DUAL;

另一個寫法:

declare

ltimestamp_start       timestamp;
ltimestamp_stop        timestamp;
linterval_diff         interval day to second;

ldt_temp               date;

begin

ltimestamp_start := systimestamp;
dbms_output.put_line(ltimestamp_start);

  -- Routine to performance test... 
  for i in 1 .. 10000 loop
    select sysdate into ldt_temp from dual;
  end loop;

ltimestamp_stop := systimestamp;
dbms_output.put_line(ltimestamp_stop);

linterval_diff := ltimestamp_stop - ltimestamp_start;

dbms_output.put_line(CHR(10)||LPAD('=',22,'=')||CHR(10));
dbms_output.put_line('  Runtime Difference'||CHR(10)||LPAD('=',22,'='));
dbms_output.put_line(
     '     Days : '||EXTRACT(DAY FROM linterval_diff)||CHR(10)||
     '    Hours : '||EXTRACT(HOUR FROM linterval_diff)||CHR(10)||
     '  Minutes : '||EXTRACT(MINUTE FROM linterval_diff)||CHR(10)||
     '  Seconds : '||EXTRACT(SECOND FROM linterval_diff) );

http://jasonvogel.blogspot.tw/2006/11/example-using-systimestamp-milliseconds.html